The present invention relates to a cathode ray tube and, more particularly, to a cathode ray tube which prevents the reflection of external light on a glass panel portion of the tube envelope, so as to raise the display contrast and prevent the formation of an electrostatic charge on the screen.
In a cathode ray tube to be used in a TV receiver or a personal computer monitor, a tube envelope in the form of a glass vacuum enclosure is used, which comprises a glass panel having a screen or an image display screen formed thereon, a neck portion housing electron guns and a funnel portion connecting the glass panel and the neck portion. A phosphor film representing the screen formed on the inner face of the glass panel is excited with modulated electron beams emitted from the electron guns to display a desired image.
FIG. 11 is a section view for explaining the structure of a shadow mask color cathode ray tube, which represents one example of a cathode ray tube with which the present invention is concerned. In FIG. 11, reference numeral 1 designates a glass panel portion; numeral 2 denotes a neck portion; numeral 3 denotes a funnel portion; numeral 4 denotes a phosphor screen; numeral 5 denotes a shadow mask; numeral 6 denotes a mask frame; numeral 7 denotes mask support mechanism; numeral 8 denotes support pins; numeral 9 denotes an inner magnetic shield; numeral 10 denotes anode button; numeral 11 denotes an internal conductive coating; numeral 12 denotes a deflector; numeral 13 denotes electron guns; and numeral 14 denotes electron beams (red, green and blue). In the cathode ray tube shown in FIG. 11, a tube envelope in the form of a vacuum enclosure is constructed of the glass panel portion 1 on which the screen (phosphor film 4) is formed, the neck portion 2 housing the electron guns and the funnel portion 3 connecting the glass panel portion and the neck portion. The inner wall surface of this vacuum enclosure is coated with the internal conductive coating 11 for supplying a high anode voltage, applied to the anode button 10, to the screen and the electron guns.
The shadow mask 5 is welded to the mask frame 6 and is suspended by the support mechanism 7 from the support pins 8, which are buried in the inner wall of the skirt portion of the glass panel portion 1, so that the shadow mask is held at a predetermined small spacing from the phosphor screen 4 formed on the inner face of the glass panel portion 1.
The inner magnetic shield 9 is provided for shielding the image display from the bad influences of external magnetic fields, such as the earth's magnetism, upon the electron beams 14 and is welded to and held by the mask frame 6.
On the neck portion side of the funnel portion 3, there is mounted the deflection coils 12 for establishing a horizontal magnetic field and a vertical magnetic field within the tube envelope, so that the three modulated electron beams emitted from the electron guns 13 are deflected in the horizontal direction and in the vertical direction to scan the phosphor film two-dimensionally and thereby to display a desired image.
Generally, this cathode ray tube is provided with an anti-reflection, anti-electrostatic charge film on the outer surface of the glass panel 1 for preventing the reflection of external light incident upon the glass panel portion or the image display screen from being reflected thereby, to prevent deterioration of the contrast of the image display or for preventing the glass panel portion from being charged with static electricity.
FIG. 12 is a section view showing, on an enlarged scale, a portion A of the glass panel portion of FIG. 11 for explaining one example of an external light anti-reflection structure of the cathode ray tube. In FIG. 12, reference numeral 42 designates a black matrix; numeral 43 denotes a phosphor screen; numeral 44 denotes a metal back; numeral 51 denotes an electron beam passing opening of the shadow mask; symbols R, G and B denote the trajectories of electron beams of individual colors; numeral 20 denotes an anti-reflection, anti-electrostatic charge film; numeral 23 denotes light emitted from the phosphor screen; numeral 24 denotes external light incident on the glass panel of the cathode ray tube; and numerals 25 and 26 denote reflected external light. The same reference numerals as those of FIG. 11 designate identical elements.
In FIG. 12, the three electron beams (R, G and B), emitted from the electron guns, are subjected to color selection for the individual phosphor dots 43 of the R, G and B colors by the electron beam passing opening 51 of the shadow mask 5 to cause them to impinge upon the proper color dots of the phosphor screen 4.
The phosphor dots 43 are excited by the impingement of the electron beams to emit light, which passes through the glass panel portion 1. The anti-reflection, anti-electrostatic charge film 20 is formed on the outer surface of the glass panel portion. The external light 25 which reaches the anti-reflection, anti-electrostatic charge film 20 of the glass panel portion 1 is suppressed in light energy through absorption or interference in the anti-reflection, anti-electrostatic charge film 20, so that normal reflection of this light toward the outer surface of the film 20 is prevented together with diffusion of reflected light 26 by the surface of the anti-reflection, anti-electrostatic charge film 20.
This anti-reflection, anti-electrostatic charge film is formed by one of various methods, but generally it is formed by the so-called "sol-gel-method."
Specifically, there is disclosed in Japanese Patent Laid-Open No. 334853/1992 a method of forming a two-layered anti-reflection, anti-electrostatic charge film by forming a film of a mixed composition in which ultra fine particles (having a diameter no more than several tens of nm) of a conductive oxide (e.g., A.T.O.: tin oxide containing antimony oxide, or I.T.O.: indium oxide containing tin oxide) for forming a high refractive index film are dispersed in an alcoholic solution, by so-called "spin-coating" to form a flat lower film having a thickness of about 60 to 100 nm, and by spin- or spray-coating the underlying film with a hydrolysate solution of silicon alkoxide to form a flat upper film having a thickness of 80 to 130 nm.
There is also disclosed in Japanese Patent Laid-Open No. 343008/1993 a method in which a film of an organic or inorganic tin compound containing antimony is formed on the glass panel of a cathode ray tube by chemical vapor deposition (hereinafter abbreviated to CVD) to form an A.T.O. film having a high refractive index, the A.T.O. film is coated flatly with a hydrolysate solution of silicon alkoxide of a thickness of 80 to 100 nm to form a film having a low refractive index, the second-layer film is spray-coated with the hydrolysate solution of silicon alkoxide to a thickness of 10 to 50 nm to form a third-layer scattering film having a low refractive index, so as to reduce the density of the reflected color exhibited by the second-layer of the anti-reflection, anti-electrostatic charge film and the reflectance in the human visible region of 400 to 700 nm, and the third-layer film is made uneven.